In the current generation of computer-controlled printing machines, interest is focused on several difficult problems.
(a) Throughput and cost--In a sense many problems flow from these two considerations, since essentially all the problems would evaporate if it did not matter how slow or expensive a printer was. In practice, marketplace pressures have made it crucially important that a printer be both competitively fast (even when printing in a "quality" mode) and competitively economical. PA0 (b) Banding and pattern artifacts--These spurious image elements are well known in lower-performance printers, but like other problems can remain troublesome, or in some cases be even more difficult to resolve, in the newer generation of devices. It is known, for example, that some banding effects can be reduced by printing highly staggered (i. e., overlapping) swaths--but also that doing so reduces overall throughput proportionately. Hence, again, high throughput tends to run counter to elimination of banding, and this conflict is aggravated by a requirement for printing at photo-like quality. PA0 (c) Liquid loading--Excessive inking is a familiar problem. To achieve vivid colors in inkjet printing with aqueous inks, and to substantially fill the white space between addressable pixel locations, ample quantities of ink must be deposited. Doing so, however, requires subsequent removal of the water base--by evaporation (and, for some printing media, absorption)--and this drying step can be unduly time consuming. PA0 (d) Prior print-mode techniques--One useful and well-known technique is laying down in each pass of the pen only a fraction of the total ink required in each section of the image--so that any areas left white in each pass are filled in by one or more later passes. This tends to control bleed, blocking and cockle by reducing the amount of liquid that is all on the page at any given time, and also may facilitate shortening of drying time. PA0 (e) Known technology of printmodes--One particularly simple way to divide up a desired amount of ink into more than one pen pass is the checkerboard pattern already mentioned: every other pixel location is printed on one pass, and then the blanks are filled in on the next pass. PA0 (f) Conclusion--Thus persistent problems of liquid loading, and pattern artifacts, countermeasured against pervasive concerns of throughput and cost, have continued to impede achievement of uniformly excellent inkjet printing. It may be added that certain combinations of these difficulties are more readily controlled on one and another printing medium; however, at least some of these problems remain significant with respect to all industrially important printing media.
As to pattern defects, the design of dither arrays or error diffusion is a logical culprit and has previously received a great deal of attention in this regard, and may be considered highly refined. Patterning originating in dither and error diffusion tends to be severe in part because a binary four-color system is capable of rendering directly such a tiny fraction of the number of colors in an eight-bit, three-primary input--i. e., only 2.sup.4 /2.sup.24 =2.sup.-20 or roughly one millionth.
To mitigate this inherent lack of tonal fidelity, dithering or error-diffusion techniques in effect trade off resolution for dynamic range. In the process these procedures intrinsically give up a very substantial amount of resolution, in the form of spatial perturbation of the original image data.
The spatial extent of such perturbations is an invitation to patterning problems. It may be somewhat less severe in multilevel than binary systems, since a system having for example five levels per color plane and six planes is capable of rendering a much greater fraction of the number of colors in an eight-bit, three-primary input than a binary four-color system. (Such a system is not considered to be prior art, but rather is associated with the development of preferred embodiments of the present invention.)
When two of the color planes are in the form of dual dilutions of a single colorant, the number of actually distinct tones is not--calorimetrically--as large as simple arithmetic might suggest. Still, such arithmetic may be helpful for purposes of general orientation, and it indicates that the directly renderable fraction of input colors is raised from a millionth (for a binary system) to 5.sup.6 /2.sup.24 or roughly one thousandth.
Yet even this fraction is small and requires some spatial dithering, or for continuous-tone input images preferably error diffusion, and heretofore some patterning persists even in images printed under conditions which should yield the best possible image quality. Theory suggests that such patterning cannot be eliminated through dither or error-diffusion redesign exclusively, and that further improvement must be sought elsewhere.
Generally speaking, tools for investigating this area heretofore have been inadequate.
In addition, if a large amount of ink is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise. These can include so-called "bleed" of one color into another (particularly noticeable at color boundaries that should be sharp), and "blocking" or offset of colorant in one printed image onto the back of an adjacent sheet with consequent sticking of the two sheets together (or of one sheet to pieces of the apparatus or to slipcovers used to protect the imaged sheet).
Still another effect is "cockle" or puckering of the printing medium. Various techniques are known for use together to moderate these adverse drying-time effects and bulk- or gross-colorant effects.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a "printmode". Heretofore artisans in this field have progressively devised ways to further and further separate the inking in each pass.
Larry W. Lin, in U.S. Pat. No. 4,748,453 assigned to Xerox Corporation--taught use of a simple checkerboard pattern, which for its time was revolutionary in dividing inking for a single image region into two distinct complementary batches. Lin's system, however, maintains contact between pixels that are neighbors along diagonals and so fails to deal fully with the coalescence problem.
The above-mentioned U.S. Pat. No. 4,965,593, which is in the name of Mark S. Hickman, teaches printing with inkdrops that are separated in every direction--in each printing pass--by at least one blank pixel. The Hickman technique, however, accomplishes this by using a nozzle spacing and firing frequency that are multiples of the pixel-grid spacing in the vertical and horizontal directions (i e., the medium-advance and scan axes respectively).
Accordingly Hickman's system is not capable of printing at on intervening lines, or in intervening columns, between the spaced-apart inkdrops of his system. This limitation significantly hinders overall throughput, since the opportunity to print such further intervening information in each pass is lost.
Moreover the Hickman system is less versatile. It forfeits the ability to print in the intervening lines and columns even with respect to printmodes in which overinking or coalescence problems are absent--such as, for example, a high-quality single-pass mode for printing black and white text.
The above-mentioned U.S. Pat. No. 5,555,006, which is in the name of Lance Cleveland, teaches forming a printmask as plural diagonal lines that are well separated from one another. Cleveland introduces printmodes that employ plural such masks, so that (unlike Hickman) he is able to fill in between printed elements in a complementary way.
It is certainly not intended to call into question the Cleveland teaching, which represents a very substantial advance in the art--over both Lin and Hickman. Cleveland's invention, however, in part is aimed at a different set of problems and therefore naturally has only limited impact on general overinking problem discussed here. In particular Cleveland seeks to minimize the conspicuousness of heater-induced deformation at the end of a page.
Thus even Cleveland's system maintains the drawback of inkdrop coalescence along diagonals and sometimes since he calls for very steeply angled diagonal lines which in some segments are formed by adjacent vertical pixels--even along columns.
Another ironic development along these lines is that the attempts to solve liquid-loading problems through printmask tactics in some cases contribute to pattern artifacts. It will be noted that all the printmodes discussed above--those of Lin, Hickman, Cleveland, and other workers not mentioned--are all highly systematic and thus repetitive.
For example, some printmodes such as square or rectangular checkerboard-like patterns tend to create objectionable moire effects when frequencies or harmonics generated within the patterns are close to the frequencies or harmonics of interacting subsystems. Such interfering frequencies may arise in dithering subsystems sometimes used to help control the paper advance or the pen speed.
To avoid horizontal "banding" problems (and sometimes minimize the moire patterns) discussed above, a printmode may be constructed so that the printing medium is advanced between each initial-swath scan of the pen and the corresponding fill-swath scan or scans. This can be done in such a way that each pen scan functions in part as an initial-swath scan (for one portion of the printing medium) and in part as a fill-swath scan.
This technique tends to distribute rather than accumulate print-mechanism error which is impossible or expensive to reduce. The result is to minimize the conspicuousness of--or, in simpler terms, to hide--the error at minimal cost.
The pattern used in printing each nozzle section is known as the "printmode mask" or "printmask", or sometimes just "mask". The term "printmode" is more general, usually encompassing a description of a mask--or several masks, used in a repeated sequence or so-called "rotation"--and the number of passes required to reach full density, and also the number of drops per pixel defining what is meant by "full density", and still further a specification of the resulting overall performance in terms such as "best quality" or "fast", or intermediately "normal".
Operating parameters can be selected in such a way that, in effect, mask rotation occurs even though the pen pattern is consistent over the whole pen array and is never changed between passes. Figuratively speaking this can be regarded as "automatic" rotation or simply "autorotation".
As mentioned above, some of these techniques do help to control the objectionable patterning that arises from the periodic character of printmasks employed heretofore. Nevertheless, for the current new generation of color printers (including high-resolution printers, or multilevel printers) generally speaking the standards of printing quality are higher, and a more-advanced control of this problem is called for.
Thus, as can be seen, important aspects of the technology used in the field of the invention remain amenable to useful refinement.